Manhole structures used in sanitary and storm water drainage systems are commonly built using stackings of precast manhole sections. Each precast manufacturer owns or can rent forms to create precast pieces of pre-defined sizes. These standard size pieces are stacked vertically to achieve the desired height and other constraints. While there may be many different configurations of the precast pieces that will achieve a desired stacking, it is usually desirable to select the configuration that provides the least expensive solution. Historically, the process of selecting manhole sections to achieve a desired stacking has been carried out manually by a person knowledgeable about the available components, their cost, availability, and other such factors. However, the selection process becomes complex as the constraints for evaluating solutions become more complex.
Precast concrete pieces are manufactured in both standardized and custom sizes and each piece can have holes cut, formed, or drilled into it as needed for the pipe inlets into the structure. While some components can be used directly from stock, many components must be customized by creating openings for pipe connections. The size, shape and position of these openings make these components custom, but they are manufactured to standard sizes and shapes where possible. Where custom size components are used, the height is the only parameter that is changed for most of the custom work.
When determining a particular manhole stacking, there are several tool constraints that normally come into play, including component diameter, height, and joint configuration. In particular, precast concrete for round manholes is typically manufactured only in certain diameters (normally 6″ increments starting with 24″ diameter). The height is also commonly stocked in standard sizes of 6″, 8″, or 12″ increments and semi-custom sizes in 1″ or 2″ increments. For virtually all manhole work, height differentials below 1″ are too small to be meaningful. The shape of the male and female fittings on the top and bottom of each piece is standardized so that pieces from different manufacturers may be mated. It is rare to use nonstandard joints.
Some aspects of the selection process are driven by the engineering specifications for the job. Typical constraints that limit the selection choices are:
1. Eccentric Cone vs. Concentric Cone—Most localities prefer eccentric cones, but some do prefer concentric cones for shallow structures.
2. Integral Base, Extended Integral Base, Separate Base—Locality preference and contractor preference will drive the selection of base, but when other considerations make the preferred base unworkable, substitutions are allowed.
3. Height of Casting/Lowest Invert—The structure should achieve the desired height within design tolerances.
4. Min. Adjustment, Max. Adjustment—The structure is normally required to have adjustability provided by the use of adjusting rings, and there is a minimum and maximum height of adjusting rings required for each structure.
5. Openings Too Close—When openings are too close to each other, the structure size must be increased to insure that there is sufficient concrete between the openings to maintain structural integrity.
6. Opening Size—The opening required on the top of the structure may be very small (24″), large (enough for a 250 hp motor), or anything in between. The choice of opening size will dictate the cones/covers that are available for use.
7. Steps vs. No Steps—Some sections are too small to allow steps to be placed on them.
8. Step Placement Technology
9. Wall Thickness—While most precast for manholes have the same wall thickness as precast pipe of the same diameter, some localities require special wall thicknesses that are non-standard.
Manhole producers typically execute several steps to provide customers with precast sections and incidental products. In general these steps are estimate and quote, structure takeoff description, shop drawing generation, bill of material (BOM) generation, and shipping and invoicing form generation.
Developing a structure takeoff description involves several steps that, if not done correctly, can lead to improper stackings and problems with the pipe connections from one manhole structure to the next. Even when done correctly, the process of generating optimal stackings can be time consuming. The process involves defining each manhole structure's relationship with other structures in the system, locating (positioning) the structures within a validated site plan, defining the structures using top of casting and bottom invert information, defining the structure's inverts, and finally, determining the best stacking for each structure. Only then can proper system drawings be produced.
It is thus a general object of this invention to provide an automated process for designing and specifying completed drainage systems and/or the individual structures used in these drainage systems.